Follow along with the video below to see how to install our site as a web app on your home screen.
Nota: This feature currently requires accessing the site using the built-in Safari browser.
I've got the same issue, the CDN for materialdesignicons has an out of date certificate - so if your forum uses SSL it won't load.
If you visit the url directly you can see the problem for yourself:
https://cdn.materialdesignicons.com/2.4.85/css/materialdesignicons.min.css